As one of the key vectors for cloning cDNAs based on the function they express in mammalian cells, we have developed a lambda phage vector that permits efficient transfection of cultured mammalian cells with a cDNA clone library constructed with the pcD expression vector. The phage vector contains a bacertial neo gene under the control of the SV40 early region promoter and SV40 RNA processing signals, as a dominant-acting mammalian selectable marker gene, and is capable of acccepting a pcD-recombiant with a maximum 9 kb cDNA insert. The recombinant phage particles are directly transfected into cultured cells after co-precipitation with calcium phosphate. Reconstitution experiments indicate that the vector is able to transduce a phenotype of a particular cDNA into one of 106 cells transfected with a library containing one functional clone of the cDNA every 105 clones. In confirming this efficiency, transfection of 10 million hypoxanthine-guanine phosphoribosyl transferase (HPRT)-deficient mouse L cells with a SV40-transformed human fibroblast cDNA library transferred into the phage vector yielded two HPRT-positive transformants that contain the diagnostic human HPRT cDNA sequences and express active human HPRT enzyme.